This invention relates to superconducting quantum interference devices used for measurement of a fine magnetic field, electric current, voltage, electromagnetic wave and so on and, more particularly, to a superconducting quantum interference device enabling high sensitivity measurement and spatial resolution but low in noise.
There is shown in FIG. 7 a structural view of a superconducting quantum interference device (hereinafter abbreviated as SQUID) according to the prior art. On a substrate 5 are integrated Josephson junctions 1, washer coils 2, a detection coil 3, and a feedback-modulation coil 4. The SQUID shown in FIG. 7 is a DC-SQUID including two Josephson junctions in a superconducting loop. The feedback-modulation coil 4 is magnetically coupled to the washer coils 2. A superconducting loop is constituted by the detection coil 3 for external magnetic field detection, the washer coils 2, and a pair of the Josephson junctions 1 is connected at the respective ends of the washer coils.
The SQUID is connected to an external control system 6 and supplied with a bias current Ib. The external control system 6 feeds back to the superconducting loop a control signal Im corresponding to a SQUID output Vs. The control signal Im is delivered as a magnetic signal to the superconducting loop through the feedback-modulation coil 4 magnetically coupled to the washer coil 2. An output voltage signal Vout corresponding to an external magnetic field, which has been detected by the detection coil 3 is obtained on the external control system.
In order to obtain high spatial resolution, the detection coil has to be made small in size. Due to this, as shown in FIG. 7 a fine detection coil 3 is integrated, together with the Josephson junctions 1 and washer coils 2, on the substrate.
Conventionally, the SQUID used for measurement on high spatial resolution has integrated Josephson junctions and washer coils and the like, together with a fine detection coil, on a common substrate. A signal generated by a signal source is detected by the detection coil. Meanwhile, a magnetic field due to the signal source links similarly to the washer coil. In such a case, measurement is made on the basic of a magnetic field linked to both the detection coil and the washer coil. As a consequence, it is difficult to measure with accuracy a magnetic field linked only to the detection coil that is to be measured.
In the case where using a detection coil having a small detection area, there is a considerable affection of a magnetic field linked to portions other than the detection coil, and particularly the washer coil.